Two Type-Safe Criteria API Proposals for JPA 2.0

Introduced two years ago as part of Java EE 5, the Java Persistence API provides a POJO persistence model for object-relational mapping. It was developed by the EJB 3.0 software Expert Group as part of JSR-220.

Persistence consists of three areas:

The API, defined in the javax.persistence package.

The Java Persistence Query Language (JPQL).

Object/relational metadata.

Whilst the JPQL significantly improved working with persistent Java objects, JPQL queries are still specified as strings. Thus whilst the queries operate over strongly-typed Java objects, they themselves are weakly typed. Building up queries in this way can be error prone, and requires specific IDE support for validation, auto completion and refactoring.

JPA 2.0, which is being developed under JSR-317 for inclusion in Java EE 6, aims to address this through the introduction of a new criteria API that offers a non-string-based approach for building queries. Expert Group lead Linda DeMichiel has published a blog entry describing the current draft of the API:

"Loosely speaking, a QueryDefinition object can be thought of as a set of nodes corresponding to the semantic constructs of the query:

Domain objects, which correspond to the range variables and other identification variables of the JPQL FROM clause

Where clause predicates, which comprise one or more conditional expression objects

Select clauses, which comprise one or more "select item" objects

Order-by and group-by items

Subqueries

and so on..."

Whilst the proposal as it stands appears to be a good step forward from the existing JPA mechanism a number of people, amongst them Gavin King, have argued that the type safety of it could and should be further improved. King, whose Hibernate O/R tool was amongst those that pioneered the use of a type-safe criteria API and was a significant influence on EJB3, has now submitted his own proposal to the Expert Group. His proposal exploits the javax.annotation.Processor introduced in Java 6 to allow a compiler plugin to build a metamodel type for each persistent class in the application. King has written two blogs posts (one, two) which describe his approach in more detail and he and his team are currently working on a prototype annotation processor for use with javac.

The Expert Group is looking at the King proposal seriously and is considering it as a replacement for the current review draft. DeMichiel told us:

"Discussion has largely focused on ensuring that this API leads to a better experience for developers, both for static queries (where the type-safe aspects should certainly shine), and for dynamic queries. We are also looking at the metamodel generation aspects."

She added that the Expert Group is very keen to hear any feedback from the development community. Please feed any comments in to jsr-317-pdr-feedback at sun dot com.

far better than strings but the code generation is really just a work around for language features that should allow for easy creation of domain specific languages. Something comparable to linq can only be accomplished with some moderate language changes.

Yes, both JPA criteria API proposals are significantly more verbose than the fluent style of API you exemplify, and at first Emmanuel and I were *really* concerned about this and were advocating for the fluent style.

However, it turns out that some of the more complex cases just don't map as elegantly to the fluent style of API, and so the JPA group made the decision to go with the current approach. I'm still not 100% happy, but now that I'm used to it, I don't think it's a really bg problem. There is definitely more typing involved, however.

Code generation... I don't like it.

That's only because youve never used a Java6 Processor :-) It's not really "code generation" in the traditional sense at all. It's an extension to the type system that is built into the compiler. This is one of those things like annotations that are scary until you use them :-)

I wonder if they considered technology like JaQu, LIQUidFORM, or Querydsl.

far better than strings but the code generation is really just a work around for language features that should allow for easy creation of domain specific languages

Yes, of course Java should have method and attribute literals. It's absurd that this was left out of the language.

But you can forget about getting them anywhere in the near future - let alone something like LINQ!

Actually I don't personally think LINQ is the right direction anyway. Better to have some more general-purpose declarative constructs in the language, that can be used to solve other problems beside "querying".

"Unfortunately that particular approach forces you to represent every persistent attribute as a public getter method, which is not a restriction that is acceptable in the JPA specification."

So I guess they will have looked at least some of themI’ve not looked at JaQu before – it looks very like writing SQL to me. What advantages does it offer over using something like raw JDBC and the Apache Dynabeans libraries?

Joe Darcy is accepting language proposals for small changes to the jdk. I don't know if "method and attribute literals" are considered small but it would be nice to see something like this incorporated into the JDK. I know that FCM (closures proposal) had something like this - I suspect BGGA did also.

From Cayenne ORM experience that had typesafe queries from day one, and recently added type unsafe JPQL, the former, is a user choice over JPQL in 95% of cases, even though Cayenne SelectQuery is missing some of the capabilities of JPQL. So yeah, great to see type safe queries coming to JPA as well - that's the way to go.

About code generation flashback. I would like to emphasize what Gavin said. There is NO build time step to generate the metamodel. Thanks to the annotation processor magic built inside javac itself, the generation is part of the compilation. you don't even change the whay you invoke javac, just being in the classpath make annotation processors being triggered.

Something I find myself wondering about is how well this works when I have some reasonably complex SQL to execute - particularly if I need to rely on vendor specific stuff (SUM, IF, FROM_DAYS, TO_DAYS etc.) or if I’m using a UDF in MySQL say. I do quite a bit of work with financial companies and I’ve always ended up reverting to raw JDBC because everything else I’ve tried seems kind of painful.